1. Field of the Invention
The present invention relates to a focus detecting device for use in a photographic camera, and more particularly to such device capable of accurately detecting the focus of a photographic optical system.
2. Description of the Prior Art
For the purpose of detecting focus in a camera, there is already known a device for guiding the light from an object onto photoelectric converting means through a focus detecting optical system and identifying, by the output signal from said converting means, whether a photographic optical system is focused at a photographic film. In such a device, a focus detecting optical system utilizing the light passing through the photographic optical system has a constant F-number corresponding to a predetermined lens aperture. On the other hand, the aperture or F-number of the photographic optical system itself is varied manually or automatically according to the desired exposure and accordingly changes the position of the optimum image surface. No difficulty arises when the constant F-number of the focus detecting optical system coincides with the F-number of the photographic optical system at the time of film exposure, but since the F-numbers are mutually different in most cases, the optimum image surface of the photographic optical system at the time of film exposure is different from the position of the film surface even if the focusing state is correctly detected by the output signal from the photoelectric converting means. Thus the conventional focus detecting device cannot be utilized for precise focusing. Other known devices utilizing a focus detecting optical system independent from the photographic optical system have had a similar defect because of a fact that the F-number at the time of film exposure varies according to the brightness of the object at the exposure time.